1. Field of the Invention
This invention relates to magnetic disk apparatuses and, more particularly, to improvement of reliability of the inter-track movement (or seek) operation of the head by a head positioning control system.
2. Description of the Prior Art
FIG. 3 is a view showing a head positioning control system for a prior art magnetic disk apparatus. Referring to FIG. 3, reference numeral 1 designates a magnetic disk. Position data written on the magnetic disk 1 is read out by a magnetic head 2 and converted by a position error signal detector 3 into a position error signal A 21 which is continuous and cyclic over all tracks and signal B 22 one-fourth cycle out of phase with respect to the signal A 21 (these signals being hereinafter referred to as position signals). The position signals A 22 and B 21 are fed to a track trace controller 4 and a track seek controller 5. When the head 2 is in an on-track state, a signal supplied from the track trace controller 4 is selected by a mode selection switch 7. When the head is moving between tracks, a signal from the track seek controller 5 is selected in the same way. The mode selection switch 7 provides a feedback control signal 28. The mode selection switch 7 is controlled by a mode control signal 30 provided from a mode controller 6. The feedback control signal 28 is converted by a power amplifier 8 into a current 29 supplied to a head drive motor 9. The head drive motor 9 generates power corresponding to the current 29 to drive an actuator 10 with the magnetic head 2 provided thereon. The magnetic head 2 is positioned to a desired position under control by the above closed loop.
The above system which uses two position signals one-fourth cycle out of phase with respect to each other is called a two-phase servo system. To form a closed loop with this system, the track trace controller 4 is required to include a compensation circuit 12 and a linear signal selection circuit 11 for selecting a signal as input to the compensation circuit 12 from among the position signals A and B and signals obtained by inverting the signals A and B. Japanese Patent No. Disclosure No. 52-4,209 discloses a typical prior art example of the linear signal selection circuit 11, having a structure as shown in FIG. 4. The operation of the circuit shown in FIG. 4 will now be described with reference to the timing chart shown in FIG. 5. As shown in FIG. 5, all the track is divided into two parts according to the level relation between the position signals A 22 and B 21. Logic signal X shows this relation. Further, with division of the track according to the level relation between the position signal A 22 and a signal obtained by inverting the position signal B 21, a logic signal Y is obtained, which is one-fourth phase out of phase with respect to the logic signal X. The combinations of the positive and negative parts of the two logic signals X and Y provide four different logic signals, i.e., linear region selection signals 25a to 25d. The process as described above is performed by the comparative operational circuit 16 shown in FIG. 4. An analog switch 15 provides only a linear position signal 27 selected by a linear region selection signal 25 from among the position signal A 22, inverted signal 24 obtained through an inverter 14, position signal B 21 and inverted signal 23 obtained through an inverter 13. In this way, a linear signal representing a position error with respect to the center of the closest track to the head 2 is used for closed loop control regardless of the head position.
In the magnetic disk apparatus as described above, the linear region selection at the time of the track trace operation is done passively by comparison of the levels of two position error signals. Therefore, if the overshoot from the desired track at the end of the seek operation as shown in FIG. 6 (showing a four-track seek) exceeds one half track, the content of the linear region selection signal is changed at this time to what selects a linear position signal showing the position error with respect to the center of the next track. Therefore, the head can not be returned to the desired track as shown by the dashed line, but is positioned to the next track as shown by the solid line. Designated at 25a to 25d are linear region selection signals at this time. In the Figure, an example is shown, in which the track seek control is switched over to the track trace control at the time of reaching of the center of the desired track. The overshoot at the time of the seek operation end is varied by external disturbances such as variations of the motor output torque and frictions and vibrations of mechanical sections. The positioning to an erroneous track (i.e., seek error) as described is conceivable in practice and constitutes a factor to reduce the reliability of the apparatus.